1. Field of the Invention
The present invention relates to a method for image processing on thermoreversible recording media and an image processing apparatus, specifically, a method for image processing capable of repetitive forming and erasing of high-contrast images at high speeds by forming high-density, uniform images and uniformly erasing images in a short period of time, and an image processing apparatus which can be suitably used for the method for image processing.
2. Description of the Related Art
Until now, forming and erasing of images on thermoreversible recording media (hereinafter may be referred to as “recording media” or “media”) are performed by contact methods in which the media are heated by contact with heat sources. Generally, thermal heads are used for forming images and heat roller and ceramic heater, etc. are used for erasing images.
Such methods for recording by contact are advantageous in being able to perform uniform forming and erasing of images by uniformly pressing the media to heat sources using platen, etc. if recording media are flexible materials such as films or paper, and making possible to manufacture image forming apparatus and image erasing apparatus inexpensively by using existing printer parts for thermosensitive paper.
However, if the recording media has a built-in RF-ID tag as described in Japanese Patent Application Laid-Open (JP-A) Nos. 2004-265247 and 2004-265249, the media becomes thick, flexibility is reduced and high pressure is needed in order to press heat sources uniformly. Moreover, if irregularity occurs on the surfaces of media, it becomes difficult to form and erase images using thermal heads, etc. Furthermore, because reading and overwriting of memory information are performed on RF-ID tag from some distance without contact, demand for performing overwriting of images from some distance has also appeared for the thermoreversible recording media.
With that, a method using a laser may possibly be used when irregularity occurred on the surfaces of media, or as a method for forming and erasing images on recording media from some distance.
Typical examples of related art which performs recording and erasing some patterns using lasers include optical discs such as CD-RW and DVD-RW, etc. On these discs, patterns as memory information are formed by the difference in optical reflectivity caused by the changes between crystalline state and amorphous state in inorganic materials such as Te, Se, In, Ag, etc. The change between crystalline state and amorphous state is caused by the difference in cooling rate after material has been melted by laser irradiation.
On the other hand, the thermoreversible recording media exhibit changes between color developing and color erasing by the difference in heating temperatures at which the media have been heated. In other words, it is necessary for the materials to be heated to their melting temperatures in a similar manner for both image forming and image erasing and patterns are formed by controlling subsequent cooling rate on the above optical discs. For the thermoreversible recording media, image forming and erasing are determined by the temperatures attained by the media due to heating by laser irradiation instead of subsequent cooling rate. Thus, processes and mechanisms of the optical discs and thermoreversible recording media completely differ from each other although same lasers are irradiated to form and erase some patterns.
Even though the difference in optical reflectivities between crystalline state and non-crystalline state of optical discs may be satisfactory for electrically detecting the difference in reflectivities by laser irradiation, the difference has been as such that it is faintly visible with eyes and is quite inadequate.
A method using lasers for forming and erasing images on recording media from some distance or when irregularity occurred on the surfaces of thermoreversible recording media is stated in JP-A No. 2000-136022, for example. It is the method by which non-contact recording is performed by using thermoreversible recording media on shipping containers used for physical distribution lines, and it is disclosed that writing is performed by using lasers and erasing is performed by using hot air, heated water, infrared heater, etc.
Methods for printing and recording using lasers are disclosed in Japanese Patent (JP-B) Nos. 3350836, 3446316, JP-A Nos. 2002-347272 and 2004-195751, for example.
The technique disclosed in JP-B No. 3350836 is an improved method for image forming and erasing which includes performing any one of forming and erasing of images on thermoreversible recording media by the heat generated from the laser beam irradiated to a photothermal conversion sheet after the photothermal conversion sheet is placed on the thermoreversible recording media. And it is disclosed in the literature that it is possible to perform both of forming and erasing of images by controlling irradiation condition of laser beams. In other words, it is stated that it is possible to control heating temperatures to a first specified temperature and a second specified temperature of the thermoreversible recording media by controlling at least one of light irradiation time, irradiated light intensity, focus and light intensity distribution or to perform forming and erasing of images entirely or partially by changing cooling rates after heating.
A method using two laser beams, in which erasing is performed by using one of the beams as oval or oblong laser, and recording is performed by using the other beam as circular laser, a method for recording using a composition of two lasers, and a method for recording using each composition of transformed two lasers are stated in JP-B No. 3446316. By these methods using two lasers, image recording of higher density than the recording using one laser can be realized.
Moreover, a technique disclosed in JP-A No. 2002-347272 in which beam shapes of laser beams are changed by optical path difference or the difference in mirror shapes by using both sides of one mirror during laser recording and erasing. By this method, it is possible to change size of light spots or to defocus by means of simple optical systems.
Furthermore, it is disclosed in JP-A No. 2004-195751 that residual images after erasing can be completely erased practically by setting a laser absorption rate of reversible thermosensitive recording media in label form to 50% or more, an irradiation energy during printing to 5.0 mJ/mm2 to 15.0 mJ/mm2, a product of laser absorption rate and printing irradiation energy to 3.0 mJ/mm2 to 14.0 mJ/mm2 and a product of laser absorption rate during erasing and printing irradiation energy to 1.1 times to 3.0 times.
In contrast, a method for erasing using lasers in which recording of clear-contrast images of high durability on reversible thermosensitive recording media is realized by erasing with laser beam energy, irradiation time of the laser beam and scan speed for pulse width which are set at 25% or more and 65% or less of those of laser recording is proposed in JP-A No. 2003-246144.
Although laser printing and erasing can be performed by the method as described above, because laser control is not operated during printing, a problem such that local heat damages occur in the places where lines are overlapped with each other or a problem of reduction in color developing density when solid images are being recorded arises during recording.
In order to settle above issues, a method for controlling printing energy is disclosed in JP-A Nos. 2003-127446 and 2004-345273.
It is stated in JP-A No. 2003-127446 that the local heat damages are reduced to prevent degradation of reversible thermosensitive recording media by lowering the energy added to the area where laser irradiation energy is controlled every draw dots to print overlapped recording dots or to print by turning back or by lowering the energy at specified intervals for printing straight.
Moreover, in JP-A No. 2004-345273, irradiation energy is multiplied by the next equation, |cos 0.5R|k (0.3<k<4) corresponding to angle R of bending point during laser drawing to reduce energy. By doing this, it becomes possible to prevent excessive energy from being added to the overlapped area of lineal drawing during laser recording to be able to reduce degradation of media, or to maintain contrast without lowering the energy too much.
Also, a method for preventing degradation of color developing density in which pitch of dot alignments in vertical scanning is set two times or more of beam diameter for color developing to make it equal to or less than the sum of diameter for color erasing and beam diameter for color developing to eliminate degradation of color developing density and occurrence of erasing marks in order to prevent erasing of images which has been recorded when overwriting is performed by lasers is proposed in JP-A No. 2004-1264.
As described above, efforts are made to prevent excessive energy from being added to thermoreversible recording media by overlapping during laser recording in the methods described as above. However, if high-density printing and uniform erasing are performed repeatedly using high-output laser, not only overlapping occurs in the area of laser drawing but phenomenon of gradually degrading thermoreversible recording media occurs even in the area of straight-line images. This is because energy distribution of irradiated laser beam becomes Gaussian distribution and energy in the center is increased excessively. The center of recorded linear image is heated excessively, deformation marks of thermoreversible recording media or generation of air bubbles are observed, and material itself, which bears color developing and color erasing properties, is thermally decomposed in the area corresponding to the center of the laser beam which is heated to high temperatures, thereby preventing satisfactory performance to be exhibited. Therefore, high-density and uniform image forming and uniform image erasing are not performed sufficiently and it is unsatisfactory as a method for recording images which is hardly degraded even when erasing/printing are performed repeatedly.
Furthermore, when thermoreversible recording media are combined with above-mentioned RF-ID tag, or pasted to bulk containers or holders, irregularity occurs on the media surfaces, making focus point of lasers inconstant, and when excessive energy is added to the thermoreversible media or even when an energy for performing erasing is added, the temperature of the media may be raised to the color developing temperature, or contrary, remainder may occur due to insufficient erasing.
Moreover, a method for recording lot numbers or model numbers directly on metals or plastics so-called laser marker is known even though it is not capable of overwriting. The laser marker forms images by melting or decomposing metals or plastics with laser energy to scratch or leave marks on the surfaces of metals and plastics. For the above method, it is necessary to focus laser and to increase the energy in the center of laser irradiation.
However, when images are formed on thermoreversible recording media, in which transparency or color tone is reversely changed by heat, by focusing laser as similar to normal laser marker, the temperature in the center of laser irradiation is increased too much, and when forming and erasing of images are repeated, the repeated area is degraded, thereby decreasing repeated numbers. And when laser irradiation energy is reduced so as not to increase the temperature of the center, size of images is reduced resulting in degradation of image contrast or prolonged time for image forming.